Until recently, most electronic circuits were manufactured using lead-based soldering material. However, in response to regulatory changes, lead (Pb) is being phased out of electronic manufacturing. For example, under the Restriction of Hazardous Substances (RoHS) directive, the European Union outlawed the use of Pb for most electronics produced after June 2006. Military and medical products may remain exempt for some time, but will eventually be subject to similar restrictions. Electronics made using lead-based soldering material is reliable and a large investment has gone into the manufacturing infrastructure that uses lead-based material for soldering electronic components. The worldwide phasing out lead-based soldering material has raised serious concerns regarding reliability of alternative methods and also raised issues of additional capital investment required to phase out certain manufacturing infrastructure.
Lead-free solder replacements in practice today use eutectic alloys such as tin-copper (SnCu), tin-silver (SnAg), and tin-copper-silver (SnAgCu), all generally referred to as Sn—Ag—Cu (SAC) material. The use of SAC soldering materials suffer from several drawbacks. For example, while lead-based soldering material has a eutectic at 183° C., SAC soldering materials have higher melting points with processing temperatures around 260-300° C. To withstand such high temperatures experienced during the soldering process, other materials in an electronic product, such as a printed circuit board (PCB) or a component packaging material is required to be more robust and are therefore typically more expensive. For example, glass transition temperature (temperature at which a material ceases to be solid and becomes flexible) of several commonly available polymers are: FR-4 (<130° C.), BT epoxy (180° C.), cyanate ester (230° C.) and polyimide (250-250° C.). The cost of the polymers with the higher glass transition temperatures tends to be more. The typical PCBs are of the FR-4 type or polymer substrate materials that have even a lower glass transition temperature than that of FR-4. In one aspect, the cost of the polymers with higher glass transition temperature is an important issue. FR-4, at glass transition temperatures higher than 125° C., can withstand a process temperature up to 230° C., which has been the proven real-world performance.
Furthermore, reliability of electronics products manufactured using SAC soldering material suffers due to uncontrolled tin whisker growth resulting in rapid and uncontrollable joint failure. The reliability issue is most critical for military and space applications where long life and reliability is of utmost importance.
In one aspect, a better technique for lead solder-free electronics is needed.